Unseen police tapes have revealed the chilling smirks of schoolgirl Becky Watts' killers as they confidently lied about the gruesome murder.

The schoolgirl disappeared without trace from her home in Bristol in March 2015.

Now as part of a new ITV documentary, Susanna Reid was given exclusive access to the hundreds of hours of footage the police filmed that reveal exactly what was going on behind the scenes of the investigation.

Becky was killed and dismembered by her stepbrother Nathan Matthews and his girlfriend Shauna Hoare in a sexually motivated attack.

As police collected alibis for everyone closest to Becky, they discovered her stepbrother Nathan and his girlfriend Shauna were at Becky’s house and were the last people to see her before she went missing.

But the cold-hearted duo put on a disturbing performance in the interrogation room - laughing and smirking as they lied about the killing.